Physiology of the Sharingan
by whoeverIamloves
Summary: One-shot (probably). Sakura has a passion for medical knowledge and a curiosity about ocular jutsus, and her Sensei just happens to have a Sharingan. Not a ship fic. (Full cover image available on DeviantArt.)


Different aspects of this one have been bouncing around in my head for a while, and so I finally took the time to connect them all in a oneshot!

My mind is prone to occasional technobabble, and the Sharingan has just been begging to be explored (in more ways than one!). :) As you read this, you might find it helpful to search the web for an eye diagram to compare.

I'd love feedback, as this is fresh from my fingertips, so if anything needs to be changed, the sooner I find out the better. I'm just too eager to get it out there, I guess. ;) I tried to keep everything consistent within the story and true to canon as possible. Let me know if you find plot holes.

Disclaimer: I don't own any aspect or component of the Naruto Universe or make any money from this.

Other Disclaimer: I have no connection to the medical field; all my expertise comes from high school anatomy class and Google Search.

* * *

**Physiology of the Sharingan**

* * *

"Sensei?" Sakura approached Kakashi's favorite tree, craning her neck up to see if he might be lounging there today.

"Hm?" He glanced down, curious what had brought her out in search of him on the team's day off.

"There you are." Sakura smiled sweetly. "Mind if I join you?"

Kakashi's eyebrow rose, but he gestured to the tree branch next to his. "Something on your mind?" he asked.

Sakura jumped up and settled herself onto the branch facing her sensei, sitting as she would on a bench rather than reclining against the tree trunk in Kakashi's style. She tilted her head to the side and pursed her lips. "Well, kind of."

Kakashi waited patiently while she searched for the right words.

"I'm curious about something," she finally said, and Kakashi nodded slowly. He lowered his book a bit, but didn't shut it yet. "I'm wondering about bloodline limits and ocular jutsus."

"I see," he murmured. "And what's brought this on?"

Sakura started swinging her feet absently. "I've been working with Tsunade every evening this week, and she's had me studying eyes and eye injuries."

Kakashi nodded again. His book lowered further as he shifted to rest his elbow on his raised knee. Eye injuries…a very personal topic for him.

"Not on any bloodline limits, though. Mainly she's had me studying and observing, and I've only been practicing a little bit, and so far only on people with normal eyes."

Kakashi's visible eye glinted with the evidence of an amused smirk under his mask, and Sakura looked away.

"Er, you know what I mean. People without any ocular jutsus or bloodline limits."

Kakashi nodded for her to continue. "So you're curious as a medic?"

She nodded emphatically. "Exactly."

He finally let his book fall closed. "So what is it that you're wondering?"

"I'm wondering a few things actually. Like, does your Sharingan drain your chakra even when you're not using it? I've always guessed that's why you keep it covered, but…you've never actually said."

Kakashi nodded. "In a way. I can't deactivate it, so in a way it's always using chakra. It uses very little when I have it closed, though. You're right; that's why I keep it covered."

"Do you feel like your other eye compensates enough? If you don't mind me asking, of course."

"I'm not sure I understand the question."

"I mean…" Sakura pressed her lips together briefly. "I mean, when you have one eye covered, like now, do you feel like your depth perception is less than it would be if you could keep both eyes open?"

"Maybe a little, but it's a very minor difference."

"Does that change at all when you use the Sharingan?"

Kakashi gave a small shrug. "It's hard to say. It's a completely different field of vision, so in a way it's useful that they overlap, but at the same time, it's not a normal depth perception as you're thinking of."

"Hmm." Sakura looked into the leaves surrounding them, musing on his answers. "I guess it might be almost impossible to compare. Because of the way chakra must flow from one eye but not the other…your brain would have to process them entirely differently, and then still make them overlap." Sakura smiled. "That's really cool, Sensei."

Kakashi just blinked. "Glad you think so." She didn't move toward the ground at all, so Kakashi waited to see what she would say next.

After a brief moment, she took a deep breath and let it out slowly. "I have another question, but you might not want to answer. I'm afraid you'll think it's kind of personal."

"Hn." He shifted from lounging back against the tree trunk so that he was sitting cross-legged on the branch. "Let's hear it, and then I'll decide."

"Well, I'm wondering…I've heard different rumors about how you got your Sharingan, but they're so mixed up that I don't know what even might be true." Kakashi kept his face carefully blank, and Sakura looked down at her stilled feet and then back up at him. "I won't ask about the details, but…did you lose your eye before you got the Sharingan? Or did you have an opportunity to get it, and then…" she wasn't sure how to finish the sentence, but she hoped he would pick up on her meaning.

"Or did I replace a natural, functioning eye?"

Sakura nodded.

Kakashi let out a slow breath before he answered. "I lost my eye first." He looked carefully at her. "I would have thought you'd have access to all of Konoha's shinobi medical files."

Sakura nodded. "Mostly, I do. I don't have access to classified mission files, though." She have a half shrug. "Your medical file just says that your teammate did the transplant in the field. Nothing else."

"I see." Kakashi's mind took him back to the mission where he'd lost his eye and gained the freshly-awakened Sharingan. The chain of events that had led to it, while useful in the sense that he'd gained a Sharingan to replace the eye he'd lost moments earlier, had been set in motion by his own foolishness.

He remembered with excruciating clarity every detail of Obito's dying moments, and he remembered Rin's shaking body and steady hands as she had carried out Obito's last wish- to give Kakashi his eye. He remembered lying still on the ground with his good eye staring straight up, fingers on either side of him digging into the dirt floor of the cave while Rin cleaned out the slashed eye. He remembered the stomach-churning sounds as she'd taken Obito's eye from Obito's body; he remembered trying not to vomit; he remembered wondering if tear ducts could work in an eye socket that held no eye. He remembered the bizarre feeling of her popping the eye into the hole; he remembered the pain of Rin using her medical tools and ninjutsu to arrange everything inside, and he remembered the pain diminishing when she cupped her hand around his temple and new eye as she healed whatever she had just arranged inside.

"Sensei?" Sakura asked gently.

"Hn." He blinked and refocused his gaze on his student.

"I'm sorry if I've asked too many questions."

His eye creased upwards in a friendly smile. "Don't worry about it. It's good that you're taking your medical training so seriously."

The corners of Sakura's mouth spread slightly in a small smile. "Thanks Kakashi-sensei…there's one other thing I was hoping to ask you."

Kakashi waited a long moment before he nodded that he was ready to hear her question.

"This is just a thought, of course," Sakura told him, "but I was hoping… Well, Konoha really doesn't have very much information on the actual science of ocular bloodline limits or jutsus. And especially not on transplants, apart from what the Uchiha clan passed down from their tradition and what Danzo did in secret. And most of the records from both of those sources have been destroyed by now."

Kakashi raised his eyebrow.

"So, my question is…would you be willing to let me find out a little bit about how your Sharingan works, and how it's connected into your body?"

Kakashi exhaled slowly and looked up at the trees. Sakura wanted to do an eye exam on the Sharingan?

"Of course you can say no. And I don't need an answer right now."

Kakashi sighed. It was always a good idea for Konoha to have as much data and research and information as possible about every subject, but he wasn't exactly thrilled with the idea of going to the hospital to have tests run, and he wasn't entirely sure he wanted anyone probing into the past where his Sharingan was concerned.

Then again, Sakura was showing real interest in medical jutsu, and he wanted to encourage her in it both as the jounin responsible for being her mentor and as a Konoha shinobi. He frowned to himself.

"What about the Byuakagan?"

Sakura shook her head. "They're still a thriving clan, and Tsunade-shishou doesn't think it would work out politically for them to want to share the secrets of the Byuakagan with outsiders." She paused. "Besides, I do plan to ask Neji or Hinata some questions too, if they're willing to answer any, but they're both out on missions for the next couple of weeks."

"Hn." Kakashi watched the leaves around them swirl in the breeze, rustling softly and causing the sunshine to shift and waver as it filtered through. "What kinds of tests would you want to run?" he finally asked.

"Nothing really invasive, if that's what you're wondering. Probably chakra-based exploration. I haven't worked out any details yet beyond that."

"I see." He paused again, then spoke without looking at her. "Have you read my teammate's report from that time?" He couldn't bring himself to call "that time" anything so clinical as "transplant" or "procedure."

Sakura shook her head. "No, I haven't. It's highly classified. I'll be given access to it if you agree to this…thing."

Kakashi had read the report when it was freshly written. It was a medical report; he'd asked to read it at one point because he was concerned about what Rin might have included. He didn't want the world to know about all the events surrounding Obito's gift if they didn't have to, but Rin's report had been clean. It had actually been very dry reading for him; it contained only the medical information about the actual procedure she'd used. It was in his medical file, he knew, but it was sealed to be inaccessible to the average medic- or anyone of a lower level than Hokage, for that matter. Even he couldn't access it unless the Hogake personally gave it to him.

"Hn."

After another long moment, Sakura shifted. "I'll go now," she said. "Thank you for hearing me out, Kakashi-sensei."

He grinned. "Anytime." He watched her jump down from her branch, wave, and then run off down the road. He idly wondered if she was on her way to study or to find her friends.

* * *

_CLASSIFIED_

_Field Procedure Report: Sharingan Transplant_

_Performed and reported by ************._

_Patient lost his left eye in combat half an hour before procedure; probable concussion moments before procedure. Donor was mortally wounded; still conscious but experiencing minimal physical sensations. _

_Patient's left eye was sliced vertically; minimal bone chipping around the eye socket. Upper and lower lids were split in two by blade._

_Patient self-treated with help from teammate. Wound was rinsed and superficially bandaged around the head; socket was not fully cleared of damaged tissue or packed._

_For the transplant, patient was supine and fully conscious. Medic cut the bandages away, cleared the socket, and irrigated thoroughly. Bleeding was not stopped in the hope that vessels would attach more easily to the new eye._

_Eye was removed from donor activated and fully intact. Medic used a medical sensory jutsu on the donor with one hand to guide from the lateral angle and the other hand to remove the eye. Upon extraction of the eyeball, a scalpel was used to access the optic nerve and remove it whole. Eye was rinsed thoroughly with saline._

_Patient's split upper and lower eyelids allowed easier access to patient's optic nerve and retinal vein and artery; medic cut away the nerve sheath and spliced the nerves together, healing the central retinal vein and artery immediately to fuse. Patient's original nerve sheath was wrapped around the optic nerve. Eye was inserted into socket. Patient seemed to experience moderate pain. Chakra binding seal was applied at the splice._

_Chakra strings were used to arrange eye muscles around the new eye at normal insertion points; acute healing jutsu was applied from anterior and lateral angles to nerve sheath, muscular insertion points, blood vessels in the socket and the eyeball, and upper and lower eyelids. _

_Emergency healing jutsu was applied to eye area for approximately three minutes. Patient reported feeling a change in the flow of chakra around the eye, and being able to feel the donor's chakra diffusing from the eye into the eye area._

_More extensive healing was performed upon return to camp while patient was unconscious. Optic nerve sheath never fully responded to medical jutsu. Patient may have moderate to high risk of future nerve damage in the case of overuse. Sheath may or may not heal on its own, given enough time and rest. _

_Some bruising visible around cheekbone; it is unclear whether this was related to the procedure._

* * *

Kakashi wandered into one of the large multipurpose rooms in the lower levels of the Hokage's office building. Three days had passed since Sakura had sought him out at his favorite readings spot, and he'd spent a lot of time since then at the memorial stone talking to Obito and Rin about whether or not to grant Sakura's request. Finally he had decided that in spite of his own discomfort with the idea, letting her do some research on Obito's nearly-extinct bloodline limit and follow up on Rin's medical accomplishment of a field transplant just might be one of the best ways he could honor his two fallen teammates. So, he'd agreed to let Sakura do her exploration on the condition that it wouldn't have to be in the hospital, and now here he was.

The room, normally containing a large round table in the center, had been cleared and now contained a few standard office chairs, a cot, a rolling cabinet, and a desk to the side. Tsunade was leaning against the desk, and Sakura was scribbling notes in a notebook as she stood leaning over the cabinet. A file folder, thick with papers, was open on the top of the cabinet beside her notebook. Kakashi guessed it must be his medical file, probably opened to Rin's report of her procedure.

Kakashi's stomach tightened at the colliding of two different worlds- it seemed strange and uncomfortable for Sakura to be taking lessons from Rin's handiwork.

Sakura looked up as he entered. "Sensei!" She beamed. "I'm almost ready. You can have a seat anywhere."

He chose a chair not too far from the rolling cabinet she was using for a writing stand and eased himself into it. He could feel Tsunade's gaze resting on him.

"Hokage-sama," he greeted her.

"Kakashi," she returned. "I'm glad you decided to come."

He grinned and scratched the back of his head. "I couldn't really see a point in saying no."

"Hn. This little exploration should give us a lot of insight into how the Sharingan works, and possibly other ocular bloodline limits as well."

He dropped the grin and nodded. "I hope it's useful information." He glanced over at Sakura to see her still jotting notes, oblivious to their conversation. "How much of this was your idea?" he asked the Hokage.

One corner of Tsunade's mouth lifted. "None. It was Sakura's idea, her initiative, her research, and her preparation. I offered suggestions here and there, but I'm only here to supervise. I wish I'd had time to do something like this sooner, but you know how things go."

Kakashi nodded thoughtfully and turned to study Sakura just as she lifted her head and put her pencil down.

"Ok, I'm ready." She looked not quite sure whether to smile or maintain a serious expression, and the resulting small smile showed both her excitement and her focus. She looked between her sensei and her mentor, silently giving each of them the opportunity to speak if either had anything to say. When both seemed to be waiting for her next move, she nodded to herself and addressed Kakashi. "Ok. There are a couple of different things I want to try, but first I just want to ask a couple of questions."

Kakashi nodded his consent, and she sat down across from him with her notebook in hand.

"Have you been sick or injured recently?"

"Nope."

"Have you been training or using a lot of chakra today or yesterday?"

"Nope. I should be at almost 100%."

Sakura smiled to herself without noticing. She glanced at her notebook, then looked up again. "When is the last time you used the Sharingan?"

Kakashi thought back. "I guess…about 5 weeks ago on a mission."

"How long did it take you to recover from that?"

Kakashi's eyebrow twitched. "I don't always have to recover from missions, you know."

Sakura blinked, then flushed slightly. "Sorry. I know. I meant to say, how long did it take you to get back to your normal chakra level?"

"Just a couple of days that time. I didn't have to use the Mangekyou, but I was using some other big jutsus."

"Ok. Um…" she scanned her notebook sheet, jotting notes about his answers so far. "How often do you use it?"

"The Sharingan, about once in every 5 or 6 missions, I'd say. The Mangekyou…apart from training, where I was using it a couple of times a week, I've only used it maybe three times ever in real combat."

Sakura glanced between him and her notebook. "When you were training, did you notice any effects of using the Mangekyou so often?"

Kakashi's visible eye creased in his signature smile. "Ah, well, I was really tired all the time. My vision in the Sharingan started getting a little fuzzy, so I stopped the training." He could feel Tsunade's heavy glare on him, probably already planning to forbid him from using the Mangekyou ever again except in truly life-and-death situations. "Now I only use it when I really have to."

Sakura was scribbling furiously. "Is it still fuzzy or has it gone back to normal since you stopped?"

"It's mostly back to normal. I only notice a difference after I've used the Mangekyou, and it's taken a little longer to go back to normal every time I've used it."

"Ok." She skimmed her page one last time, then lowered it and looked at him. "That's all my questions for now. Next, I want to use a jutsu to just check how everything is arranged inside, if that's ok with you."

"Hn." He lifted his forehead protector to reveal the eye, but kept it closed as he waited for instructions.

"First I want to check your right eye, just as a comparison." She formed a single seal with her hands, and then raised her hand to his right temple where it began to glow a very soft green. "Close your eyes," she requested.

Kakashi obeyed, and Sakura closed her own eyes too as she used the green glow as an extension of her senses to feel around without touching anything. She noted that the nerves and muscles were more developed than in most of the patients she'd treated so far for vision issues, probably from using it double the amount as most people, since most people have two average eyes. She made a mental note to find someone who truly had only one eye and see if the same was true of that person.

She dropped her hand and raised the other to his opposite temple. "I'm switching now," she told him, and then that hand began to glow the same soft shade of green.

Kakashi was surprised when she was done so quickly with checking his right eye. He had hardly felt anything at all from her exploration, which was a surprise, but a welcome one. The most he had noticed was a tiny amount of pressure moving quickly around in his eye and behind it—barely enough to be aware of, except that he was paying close attention to it.

Sakura used her chakra to feel ever-so-gently around the shape of the Sharingan, searching for the connection points of Kakashi's own eye muscles and blood vessels. She noted that some vessels in the eye had stayed in the eye, never connecting to outside vessels, while other vessels had branched to connect to those already in place from the natural eye that had previously occupied the space. She also noted that there were slightly fewer muscles connected to the Sharingan than there were to Kakashi''s right eye, but those that were connected seemed to only be of average development.

"It's interesting," Sakura murmured. "This eye doesn't seem to get as much exercise as the other eye."

Tsunade tilted her head slightly. "That's to be expected, I'd say."

Sakura's forehead scrunched slightly. "Well, in a way, yes, but the muscles around this eye seem almost underdeveloped compared to the other eye. I thought they'd at least be equal, since a person's eyes usually move together even when one is covered."

Tsunade nodded thoughtfully, and Kakashi absorbed the information with a slight lift of an eyebrow.

"Implication?" Tsunade prompted.

"Probably…I'd guess that it still moves in tandem, but maybe doesn't have to focus the way the other eye does?"

Tsunade's lips showed a hint of a smile. "Sounds plausible."

"Sensei, this might be a small part of the reason you get tired faster when you use the Sharingan. Obviously the chakra drain is a big part, but maybe if you started exercising this eye more, it might not need to drain quite so much chakra to keep up with your techniques."

Kakashi's open eye opened a bit wider, impressed that Sakura's exploration was yielding practical results so soon.

Sakura fell silent again and moved her chakra inwards. She felt around for all the normal parts of the eye, and she was mildly surprised to find each one as she would in a normal eye.

A few of the components of the Sharingan were a bit different from the normal version, though. The most obvious difference was the high concentration of chakra vessels through the cornea, iris, lens, and body of the eye. The iris itself actually contained a solid ring of chakra vessels, concentrated at each tomoe, and it was these vessels that actually created the tomoe pattern and connected them to each other. The outer parts of the eye, such as the sclera, were actually thicker than in Kakashi's natural eye—probably to insulate the chakra flow and protect the eye more, Sakura surmised.

"It's really amazing," Sakura said after a moment. Kakashi opened his non-Sharingan eye to look up at her, and could see Tsunade watching intently as well. "Your teammate must have been a really great medic to be able to pull this off so successfully."

"Hn," he agreed softly. "She was."

"There's so much precision here…I can't imagine trying to do that kind of surgery under the pressure of an emergency field situation." Kakashi's lips pulled themselves into a small, proud smile under his mask. He was sure that given the right situation, Sakura would actually be capable of performing intricate emergency surgeries, but it made him feel good to hear her praise Rin that way.

She felt her way back toward the optic nerve, gently feeling the sheath for the point where his teammate had opened it to splice it together. She found the seam, and as she brushed her chakra along it, she felt Kakashi tense slightly. She immediately lightened her chakra's touch and opened her eyes to look at Kakashi.

"Did that hurt?"

Kakashi relaxed again. "No…I just felt it more than I have yet. More noticeable."

"I see." She paused for a moment, thinking about the implications. The seam wasn't raw or particularly thin; it was just thinner than the rest of the sheath. She nodded. "There's just one more thing I want to check. I'll try to be as light as possible. Tell me if you feel any discomfort at all."

"Hn."

Sakura slowly eased her chakra back to the seam, and as lightly as she could, she traced it from the beginning to the end, where the sheath then smoothed into his own natural nerve and linked into the rest of his nervous system.

She went back into the eye and found where the main artery and vein fed into the body of the eye, and she very slowly and cautiously began to trace them through the inside of the sheath along the optic nerve as Tsunade had suggested to her during their planning session. She kept her own eyes open this time, watching Kakashi carefully for any sign of discomfort.

He showed no reaction, and her chakra met no resistance as she followed the vein out of the eye. Soon, she reached the point where the Sharingan had been spliced to Kakashi's own system, and she stopped there to take note of how it had been done and how it had healed.

The cuts had been clean on each end, both from the eye and from Kakashi's own body. Sakura could feel a ring of scar tissue at each connection point, presumably where his teammate had neatly tucked one end into the other with a slight overlap for both the artery and the vein.

That must have been a judgment call, Sakura realized, because the report had said the procedure was definitely an emergency one. In a hospital, there would have been time to simply place the ends next to each other and let them heal together, resulting in less scar tissue, but in a hospital, a patient wasn't likely to have to jump right back into battle. Overlapping them in addition to binding them with the acute healing jutsu would have made the connection much more secure, even if it was a little less ideal for the blood flow to and from the eye as Kakashi got older.

Sakura slowly backed out of the nerve, pulling her chakra back into her hand as carefully as she'd sent it out. When she was completely disconnected from Kakashi, she sat back in her chair and let out a long breath. She grinned. "That was really fascinating," she said.

Kakashi raised his eyebrow with an amused glint in his open eye. "I'm glad you're having fun," he teased.

"Sakura. Notes," Tsunade ordered in a stern voice, even as she smiled. Sakura gave a quick "hai!" and reached for her notebook. She started scribbling, and Kakashi, curious, opened his Sharingan to watch her pencil scratch and see what she was writing. A lot of the medical jargon didn't make much sense to him, but he found himself curious about the sketches. He made a mental note to ask her about them later, after she'd had a chance to process her thoughts a little more.

After a few long moments with no sound except the fast scratching of Sakura's pencil, her hand finally stilled and she quickly skimmed what she'd just written. Kakashi closed the Sharingan just as she looked up.

"Ok, now for something a little different." She looked to Tsunade for her approval, and when the Hokage nodded once, Sakura turned back to Kakashi. "You can change your mind at any time if you want," she reminded him, and he nodded but said nothing. "What I want to do next is a justu that creates a map of your chakra system in the air while also showing the nuances of chakra types."

"I see."

She reached around the cabinet and pulled out a scroll. "Here's how it works. The jutsu is already prepared here in this scroll; it just needs to be spread out. Once that's done, you would lie at the center, and I'll finish the jutsu by raising one twentieth of your current chakra into the air above you. The jutsu will hold it in place as a real-life model image and make it visible."

She stopped to let him absorb her explanation, then asked with only a small trace of the hopefulness she felt, "So what do you think?"

Kakashi pressed his lips together under the mask and sat back in the chair. "How does it release?"

"When I let it go, that chakra will dissipate completely. I can do that either into the air, or back into your system." She paused. "You shouldn't feel anything except a slight tug when I actually lift the chakra."

She paused before finishing her explanation in a voice she hoped sounded confident and matter-of-fact. "Assuming everything goes smoothly with that part, the last thing I want to try is to have you cast a really simple, harmless genjutsu on someone so that we can see exactly how your chakra flows. Tsunade-shishou has hand-picked someone with a high enough clearance level for the small amount they'd see of what we're doing here, if you're willing to try it."

Kakashi nodded slowly. "I see." He glanced as discreetly as he could over to the Hokage, who was also watching him very closely. She gave no indication one way or the other, though, so he looked back to Sakura with his signature grin. "Ok. Let's see what you've got then."

"Hai!" Sakura immediately stood up and walked to the center of the room, unsealing the scroll as she walked.

Kakashi lowered his voice and looked over at Tsunade. "Has she done this before?"

"Twice," she told him, then looked back at Sakura. "Never on a conscious person, though."

The skin between Kakashi's eyebrows crinkled slightly. "Does that make a difference?"

The Hokage gave a short, soft chuckle. "It shouldn't, as long as you stay still."

"…hn."

"The first one was actually more difficult than this one. The slightest slip of control on her part, if it happened at the wrong moment, could have been dangerous."

"Then I guess it's a good thing that's her strongest area."

"That's the only reason I approved her request for all this," Tsunade agreed.

Both fell silent as they watched Sakura kneel in the center of the room, spread the scroll out, and form a few hand seals before planting her hand solidly in the center of the paper. Writing shot in every direction from it, fanning out into a square and diamond overlay that covered the floor 2 meters in every direction from where she had planted her hand. She held her position until the writing settled, and then she stood and Kakashi followed suit.

"Kakashi-sensei, over here please?" As he approached, she gestured the direction she wanted him to lie: with his shoulders across the open scroll. He obliged, and she knelt at his shoulder. "Ready?"

"Hn," he murmured from his position lying on his back.

"I need you to keep your body very still," she added, then formed three more seals with her hands. He closed his eyes in response.

She locked her eyes on his chest, then slowly, maintaining half of a seal with her left hand, she reached out with her right and tapped the center of his chest very lightly with her fingertips. She stayed frozen like that for several seconds, then began to lift her hand. As she lifted it, her fingertips drew out a perfect replica of the shape of Kakashi's body, only it seemed to be made entirely of shimmering blue wires. One corner of her lips lifted absently at the thrill of her accomplishment.

She lifted the wire-frame model of her sensei until it was at her eye level. "Sensei," she whispered, "you should look."

Kakashi opened his non-Sharingan eye and felt a sense of awe run through him. His eye widened at seeing such a bright, translucent, fleshless version of himself, and especially at the way the chakra looked like it was pulsing through his system exactly how it was probably circulating inside his actual body. He exhaled slow and deep, and he was shocked to see the wire frame above him do the same.

Sakura brought her right hand to rejoin her left in maintaining the seal, and she watched Kakashi's expression change with amusement and the slightest hint of pride. While he stared at the model above him, she glanced at his floating form just long enough to confirm that nothing seemed out of the ordinary in most of his chakra system, then she shifted her attention to her true interest- the Sharingan.

Nearly every vessel that composed the Sharingan was pulsing with a chakra that was somewhere between red and orange, with the occasional extra-fine blue line weaving itself into the pattern. Sakura could see the many of the shapes she had felt only moments ago. "The Sharingan is still active with the original chakra of the…of its original owner."

Kakashi's gaze drifted over to meet hers, then back to the hovering red-orange representation of the Sharingan floating above him.

Sakura's eyebrows drew together as she noticed a third color of chakra at the place where she'd explored the optic nerve sheath and the connections inside it just moments ago. "It's…there's another," she murmured absently. This one was a deeper color, a purple that was almost brown, and there was notably less of it than of the red-orange chakra. Sakura's eyebrows rose high when she realized what she was seeing.

"Sensei, do you see this dark purple here?" she whispered. He nodded his head very subtly to indicate that he did, and the form above him mirrored the movement. "Your teammate- the medic- must have left this when she did the transplant."

Kakashi exhaled deeply again, eyes wide and face strained as he stared at the spot Sakura had indicated. Sakura continued after a moment of thinking. "The report mentioned a binding seal to link the eye into your nervous system, but I thought it must mean just a physical bind to keep the parts from coming loose. Now, though…" she took a slow breath in and let it out again, "Now, seeing this, I think that binding jutsu must function not only as a sort of glue, but also as a converter to let your body read the chakra signals from the Sharingan and to let the eye use your chakra for jutsus."

Kakashi swallowed, and the room was silent for a few moments as they both absorbed the new information and each tried to reconcile it with what they already knew- Kakashi with what he remembered of Rin performing the operation in that battlefield cave, and Sakura with what she knew of the way chakra interacts and flows through a person's system.

Sakura seemed to finish her musings first, because after a moment she interrupted his thoughts.

"Sensei?"

He tore his gaze from the tiny purple-brown tendrils above him to acknowledge her.

"I'd like you to focus your chakra to one hand, as if you were going to mould it."

Both watched as the blue wires got slightly dimmer in the rest of his body at the same rate the glow intensified in his left hand. He held it there for a moment until Sakura murmured for him to let it go, and almost immediately the brightness was evenly redistributed throughout the model.

"Ok, now…open the Sharingan," she instructed quietly. Kakashi did so, and immediately the few small blue lines weaving through the eye doubled in brightness. Sakura's eyes widened again, and she stared intently for several seconds as she tracked the flow of chakra. She watched as it entered the eye through the purple-brown splice, circulated through each part of the eye, and pulsed back through the nerve splice toward the hovering body's central nervous system.

Finally, she breathed out and returned some of her focus to the rest of the room. "Shishou?" She asked, still watching Kakashi as he stared upwards, lost in his thoughts about the Sharingan he could see mirrored above him.

"You're ready?" Tsunade responded.

"Hai," Sakura breathed.

Tsunade went to the door and opened it, saying nothing but gesturing for whoever was waiting outside to enter.

"Kakashi-sensei?"

"Hn," he muttered absently.

"It's time for the last part."

He looked over at her, then over to his other side where she directed his gaze. He was a bit surprised to see Genma standing there, senbon-less for once, but wearing his usual blank expression as he surveyed the blue wire form hovering above Kakashi.

"You good?" Genma asked.

"Hn," Kakashi confirmed.

"Sensei," Sakura said, and both jounin turned their eyes to her, "I want you to cast a simple false surroundings genjutsu on him."

Kakashi looked back over at his friend. "You good?" he asked with a hint of hesitation in his voice.

Genma nodded nonchalantly and lowered himself to one knee beside Kakashi. "Yep, I'm good. Just no mindfucks, please."

Kakashi smirked to himself, which was visible to the others from the slight twitch around the mouth of the floating chakra model. Then he glanced back at Sakura for the cue to begin.

Sakura locked her eyes on the model of the Sharingan and raised one of her hands to block the line of sight between here eyes and the real Sharingan in order to avoid getting caught in the genjutsu along with the volunteer.

"Genma, please don't release the genjutsu. One of us will do that for you when it's time. Sensei, you can start whenever you're ready," she said.

Kakashi shifted his gaze back to meet Genma's, and the other jounin gave him a tiny nod of encouragement signaling his consent and readiness. Kakashi sent him into a swirl of leaves, and the next moment he knew Genma would be looking around him at a bunch of trees in the middle of an imaginary forest.

Sakura watched as the iris of the red-orange eye seemed to start rotating, and as she looked closer she realized the rotation was actually the pulse of the concentrated chakra in each of the tomoe as it pulsed in an ever-accelerating circuit. She saw the spherical body of the eye fill with blue chakra until there were almost equal parts of both colors, and then the blue began to funnel out of the eye through the center of the iris in long, thin, rotating tendrils reaching in the direction of Genma. As it got farther from Kakashi's body, it faded from the model, but Sakura watched intently as strands of chakra continued to be emited from the body of the eye and spun into graceful threads by the spiraling tomoe of the Sharingan's iris.

Finally, just when Kakashi was beginning to wonder if he'd caught her in the illusion as well, she broke the silence. "You can stop now," she said softly, and Kakashi let the eye go back to normal before he closed it. When he did, the remaining bright blue chakra in the eye gradually began to dissipate back through the purple-brown link.

"I'm going to lower the model back into your body now," Sakura told him, and he exhaled as he nodded. "Be extra still."

Sakura returned her fingertips to the center of the floating form's chest and gently guided it down. It glided smoothly through Kakashi's clothes and skin, just as it had when she had pulled it out, until her fingertips were once again resting on his actual chest. After a few seconds, she flattened her hand in the same spot, fingers together and palm touching the fabric of his vest. Then, suddenly, her fingers splayed out with an abrupt "Kai!" and Kakashi felt no more chakra connection between her hand and his body.

Almost as abruptly, the writing on the floor evaporated into the air. As the last traces of it vanished, Sakura finally let her concentration break. A grin broke out on her face, and she took a deep breath and let it out again quickly.

"That's all, Kakashi-sensei," she said, and he blinked once or twice and glanced around one last time before he sat up.

Genma was still staring off into nowhere, and Sakura gestured to Kakashi with raised eyebrows to silently ask if he wanted to release the genjutsu or if she should do it. Kakashi nodded and formed half a seal, then muttered "kai!" under his breath. Genma blinked and quickly refocused on the two people in front of him.

"Hm," he breathed. "That was a pleasant little vacation."

Sakura smiled, and Tsunade dismissed the other jounin. Sakura stood up first and looked to offer a hand to Kakashi, but he was already up by the time he would have taken it and tugging his forehead protector back into its normal position. They headed back toward their original seats as Genma trotted toward the exit with a quick "see you around."

As soon as he was gone, Tsunade spoke again. "I've got to be going too," she said. "Sakura, come find me once you've done your analysis and we'll go through it. Kakashi, thank you for helping Konoha understand a little more about the Sharingan."

"Hai," Sakura answered, and Kakashi nodded, and then the Hokage was out the door too.

"Do you have any questions for me, sensei?" she asked him.

Kakashi looked around the mostly empty room. "Not really…I'm curious to know what all you found out at some point, though. Especially about the binding jutsu."

Sakura nodded. "I'll let you know as soon as I'm able to collect all my thoughts about it and talk to Tsunade-shishou."

"Hn," he murmured.

"And thank you for being my patient, sensei. I'm really excited about everything I've learned from today."

He studied her for a long moment her with his uncovered eye. "I should be thanking you. Sakura, you showed me a very precious gift that I never even knew I'd received." Sakura looked away, not sure how to take his unusual openness, but then his eye revealed his standard exaggerated grin. "Ah, anyways, I'm actually pretty impressed with your skills here. Keep up the good work."

Sakura smiled. "Hai," she said, and then started to collect her notebook. "I'm going to be in the library for a while…I really want to get all this compiled while it's fresh on my mind."

Kakashi nodded. "Ok. See you at training tomorrow?"

"M-hm!" Then Kakashi was gone, and Sakura finished gathering up her things so she could go organize and record her discoveries from the afternoon.

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**END**

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